A requirement has arisen in recent years for high-quality reproduction of both printed images and continuous tone images by utilizing certain types of display device, such as a gas plasma display, which are inherently capable of generating only two levels of display density. This is achieved by generating a spatial gray scale display image. However problems are presented by the prior art methods proposed for implementing such reproduction.
WIth such a bi-level display device, each picture element generated by the display can be set (e.g. as an element of a dot matrix) in either a bright or a dark state. There have been various proposals for implementing pseudo-continuous tone reproduction by such display devices by employing a spatial gray scale. These methods are based on setting a relatively high proportion of picture elements of the display in the light state to represent a light-density region of the original image, and a high proportion in the dark state in the case of a region of the image which should appear dark.
The most well-known method of providing such pseudo-continuous tone reproduction is the dither technique. With this method, a continuous tone image is reproduced on the basis of numbers of dots within each of predetermined areas of the continuous tone image, and utilizes a dither matrix. Threshold values of the dither matrix are compared with the level of an input signal, one picture element at a time, to thereby execute bi-level image conversion processing. However this method has the disadvantage that the continuous tone characteristic and the resolution of the image that is obtained will depend directly upon the size of the dither matrix, and have a mutually incompatible relationship. Moreover with the dither technique, it is difficult to avoid the generation of moire patterns, particularly when printed images are reproduced.
A method has been proposed in the prior art which is highly effective in overcoming the problems of the dither method, i.e. reducing the incompatibility between a good continuous tone characteristic and high resolution, and suppressing the generation of moire patterns. The method is called the error diffusion method, which has been proposed by R. Floyd and L. Steinberg under the title "An Adaptive Algorithm for Spatial Gray Scale", published in the SID 75 Digest, pp 36-37. The basic principles of the proposed method are as follows. To determine whether a picture element of the input signal is to be displayed at the dark or at the light level, the corresponding level of the input image signal is compared with a threshold value and a light/dark decision thereby made. The density of a picture element of the display will in general be in error with relation to the desired density of that element, i.e. the density of the corresponding picture element in the original continuous tone image. This error is referred to in the following as the bi-level conversion error. However the effect of this error are substantially reduced by subsequently modifying the respective values of desired density of a set of picture elements which are positioned peripherally adjacent to the object picture element.
With the method proposed by Floyd and Steinberg, the effects of the bi-level conversion error of each picture element are substantially reduced by modifying the respective desired densities of a set of picture elements which are positioned peripherally adjacent to that picture element, the modification being performed by apportioning the the value of error obtained for that object picture element among these peripheral picture elements, with the error being apportioned in accordance with predetermined fixed factors. Such factors are referred to in the following as apportionment factors.
The error diffusion method is superior to the dither method, with regard to image resolution and continuous tone characteristic, and enables the degree of generation of moire patterns to be made very small, even when a printed image is being reproduced. However in the case of reproduction of an image which has only small amounts of changes in density, and computer-generated images which have areas of extremely uniform density, the error diffusion method produces regions of texture in the reproduced image, with this texture being specific to the error diffusion method. For this reason, the error diffusion method has not been widely adopted. The reason for generation of this texture is that a fixed relationship is continuously maintained between an object picture element and the aforementioned set of picture elements which are disposed peripherally adjacent to the object picture element, and fixed values are also maintained for the respective proportions by which the bi-level conversion error of the object picture element is apportioned among these peripheral picture elements.
The term "object picture element" as used herein has the significance of a picture element which is currently being processed, to determine a corresponding bi-level display image value, with data being utilized in that processing which have been obtained beforehand during processing of preceding picture elements.